1. Technical Field
Aspects of the present invention relate to a communication apparatus and in particular to a communication apparatus that controls to suppress an oscillation in a line communication network when the communication apparatus is connected with a various kind of networks.
2. Description of Related Art
In order to maintain communication quality, a public line network is designed in accordance with predetermined standards. A communication apparatus connected to such a network is configured such that a line voltage or a line current is controlled so as to meet the standards, since a line resistance connected with an exchange installed in a telephone exchange station varies depending on the location of the communication apparatus.
For an example, Japanese Patent Provisional Publication P2004-112490A describes a communication apparatus that detects a line current and a line voltage, calculates line impedance, and adjusts the signal level of the modem based on the calculated impedance. Further, in order to control the line current and the line voltage which meet the standards of public line networks, etc., the apparatus adjusts a relation between the current and the voltage so as to satisfy the standards, based on a predetermined DC mask curve defining a relationship between the line current values and line voltage values. FIG. 10 is a graph showing an example of the DC mask curve used in a conventional communication apparatus.
In FIG. 10, a horizontal axis represents the line current value, while a vertical axis represents the line voltage. An area between two dotted lines represents line current values and line voltage values satisfying the public line standards.
A predetermined DC mask curve passing the above area is defined (indicated by solid line in FIG. 10), and by controlling the impedance of the line, the line current and the line voltage are adjusted.
For example, in FIG. 10, when initially a line current value and line voltage value are represented by point A, which is out of the area, point A is located below the DC mask curve and the values do not meet the standards. In such a case, the value of the line impedance is changed to a smaller value. Then, the line current value and the line voltage value are changed to values represented by point B. In this example shown in FIG. 10, point B is also out of the area (i.e., located above the DC mask curve). Therefore, the value of the line impedance is then changed to a larger value, and the line current value and the line voltage value are changed to point C. As shown in FIG. 10, still point C is out of the area (i.e., located below the DC mask curve), and the value of the line impedance is changed again to a smaller value. Then, the line current value and the line voltage value move to point D, which is substantially on the DC mask curve. The line current value and the line voltage value set in this way satisfy the standards.
However, when the line current value and the line voltage value are set in the way described above, an oscillation may occur as the network is in the resonant state with specific frequencies. The oscillation frequency is only within a frequency bandwidth from 20 kHz to 1 MHz, which is higher than an audio frequency range. However, the oscillation generated in the network would cause troubles such as generation of noises, malfunction in echo canceling while calling with a handset.